First U.S. 6-Bundle, 745-kV Line Requires Large Pulling Blocks

Transmission construction is a specialized type of line installation. Not only are structures taller, spans longer and conductors larger, but the pulls are longer and go over very arduous terrain. Conductors vary, requiring special handling techniques, special tools and equipment.

In the early design stages of its unprecedented 6-bundle, 765-kV transmission line (now being constructed in Virginia and West Virginia, U.S.), AEP retained Sherman & Reilly to design and manufacture the special-built 6-conductor bundle blocks with helicopter attachment and block grounding capability.

Stringing Block Description

The blocks are 32 inches in diameter with 6 conductor sheaves and one center pulling line sheave with all grooves polyurethane lined and mounted on ball bearings for stringing 6 subconductors simultaneously with one common pulling line.

Blocks are designed to accept the “pilot rope” from helicopter “fly-in” to pull the main pulling “hard line rope” back to the conductor tensioner end prior to pulling the 6 conductors using a 6-conductor equalizing running board with pulling swivels.

Transmission line construction also brings different line and structure designs, conductor configurations (single and bundle two, three and four per phase). In the case of AEP's 90-mile, 765-kV line from Wyoming Substation in Wyoming County, West Virginia, to AEP's Jackson Ferry Substation in Virginia the bundle totals six. A few companies, including AEP, have done trials but this is the first to go on-line in the United States.

Helicopter Attachment

This special block's exclusive helicopter attachment feature is designed to keep both the pilot rope and the hard line rope captive until such time the 6-subconductors arrive at each tower and block for an even and smooth transition of the conductors through each stringing block.

Block grounds are furnished on every other block to ensure direct grounding between the subconductors or pulling hard line rope and the towers, a counterpoise system, etc. where high-induced voltage from other adjacent energized lines could be experienced.

Grounds are designed to operate on an acentric allowing for sag differences in the 6-subconductors during the stringing operations, thus assuring positive contact on the 6-subconductors at all times.

Block components, except for the frame, are made from high-strength aluminum alloy for maximum capacity, yet are lightweight so blocks can be flown-in by helicopter and handled in rugged mountainous terrain. Blocks also are designed to be used when pulling 2 or 4 conductors as well as the 6-conductor bundle configuration.

Sherman & Reilly has a 40-year history of working with the conductor manufacturers, the utility construction departments, design engineers, consultants and line contractors to develop the necessary tools and equipment to keep abreast of the changing needs and problems of installation.